Claims:

1. An integrated hydrocracking process for producing cracked hydrocarbons
from a feedstock including: a. separating the hydrocarbon feed into an
aromatic-lean fraction and an aromatic-rich fraction; b. hydroprocessing
the aromatic-rich fraction in a first vessel of a first stage
hydroprocessing reaction zone to produce a first vessel first stage
hydroprocessing reaction zone effluent; c. hydroprocessing the
aromatic-lean fraction in a second vessel of the first stage
hydroprocessing reaction zone to produce a second vessel first stage
hydroprocessing reaction zone effluent; d. fractionating a mixture of
first vessel first stage hydroprocessing reaction zone effluent and
second vessel first stage hydroprocessing reaction zone effluent to
produce one or more fractionating zone product streams and one or more
fractionating zone bottoms streams; e. hydroprocessing at least a portion
of the fractionating zone bottoms stream in a second stage
hydroprocessing reaction zone to produce a second stage hydroprocessing
reaction zone effluent; and f. conveying the second stage hydroprocessing
reaction zone effluent to the step of fractionating.

2. The method of claim 1, wherein the first vessel first stage
hydroprocessing reaction zone is operated under relatively severe
conditions effective to remove heteroatoms from, and to hydrocrack, at
least a portion of aromatic compounds contained in the aromatic-rich
fraction.

3. The method of claim 1, wherein the second vessel first stage
hydroprocessing reaction zone is operated under relatively mild
conditions effective to remove heteroatoms from, and to hydrocrack, at
least a portion of paraffin and naphthene compounds contained in the
aromatic-lean fraction.

4. The method of claim 1, wherein the second stage hydroprocessing
reaction zone is operated under relatively mild conditions effective to
remove heteroatoms from, and to hydrocrack, at least a portion of
paraffin and naphthene compounds contained in the fractionating zone
bottoms stream.

5. The method of claim 1, wherein the aromatic-rich fraction includes
aromatic nitrogen compounds including pyrrole, quinoline, acridine,
carbazole and their derivatives.

6. The method of claim 1, 1, wherein the aromatic-rich fraction includes
aromatic sulfur compounds including thiophene, benzothiophenes and their
derivatives, and dibenzothiophenes and their derivatives.

7. The method of claim 1, wherein separating the hydrocarbon feed into an
aromatic-lean fraction and an aromatic-rich fraction comprises:
subjecting the hydrocarbon feed and an effective quantity of extraction
solvent to an extraction zone to produce an extract containing a major
proportion of the aromatic content of the hydrocarbon feed and a portion
of the extraction solvent and a raffinate containing a major proportion
of the non-aromatic content of the hydrocarbon feed and a portion of the
extraction solvent; separating at least substantial portion of the
extraction solvent from the raffinate and retaining the aromatic-lean
fraction; and separating at least substantial portion of the extraction
solvent from the extract and retaining the aromatic-rich fraction.

8. An integrated apparatus for processing heavy hydrocarbon feedstocks to
produce clean transportation fuels comprising: an aromatic separation
zone operable to extract aromatic organosulfur compounds from the
hydrocarbon feed, the aromatic separation zone including an inlet for
receiving the hydrocarbon feed, an aromatic-rich outlet and an
aromatic-lean outlet; a first vessel of a first stage hydroprocessing
reaction zone having an inlet in fluid communication with the
aromatic-rich outlet and an outlet for discharging first vessel first
stage hydroprocessing reaction zone effluent; a second vessel of the
first stage hydroprocessing reaction zone having an inlet in fluid
communication with the aromatic-lean outlet, and an outlet for
discharging second vessel first stage hydroprocessing reaction zone
effluent; a fractionating zone having an inlet in fluid communication
with both the first stage hydroprocessing reaction zone effluent and a
second stage hydroprocessing reaction zone effluent, one or more outlets
for discharging product and one or more outlets for discharging
fractionating zone bottoms; and a second stage hydroprocessing reaction
zone having an inlet in fluid communication with the fractionating zone
bottoms stream, and an outlet for discharging second stage
hydroprocessing reaction zone effluent.

Description:

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent
Application No. 61/513,233 filed Jul. 29, 2011, the disclosure of which
is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to hydroprocessing systems and
methods, in particular for efficient reduction of catalyst-fouling
aromatic nitrogen components in a hydrocarbon mixture.

[0004] 2. Description of Related Art

[0005] Hydrocracking operations are used commercially in a large number of
petroleum refineries. They are used to process a variety of feeds boiling
in the range of 370° C. to 520° C. in conventional
hydrocracking units and boiling at 520° C. and above in the
residue hydrocracking units. In general, hydrocracking processes split
the molecules of the feed into smaller, i.e., lighter, molecules having
higher average volatility and economic value. Additionally, hydrocracking
typically improves the quality of the hydrocarbon feedstock by increasing
the hydrogen to carbon ratio and by removing organosulfur and
organonitrogen compounds. The significant economic benefit derived from
hydrocracking operations has resulted in substantial development of
process improvements and more active catalysts.

[0006] Mild hydrocracking or single stage hydrocracking operations,
typically the simplest of the known hydrocracking configurations, occur
at operating conditions that are more severe than typical hydrotreating,
and less severe than typical full pressure hydrocracking. Single or
multiple catalysts systems can be used depending upon the feedstock
processed and product specifications. Multiple catalyst systems can be
deployed as a stacked-bed configuration or in multiple reactors. Mild
hydrocracking operations are generally more cost effective, but typically
result in both a lower yield and reduced quality of mid-distillate
product as compared to full pressure hydrocracking operations.

[0007] In a series-flow configuration the entire hydrocracked product
stream from the first reaction zone, including light gases (e.g.,
C1-C4, H2S, NH3) and all remaining hydrocarbons, are
sent to the second reaction zone. In two-stage configurations the
feedstock is refined by passing it over a hydrotreating catalyst bed in
the first reaction zone. The effluents are passed to a fractionating zone
column to separate the light gases, naphtha and diesel products boiling
in the temperature range of 36° C. to 370° C. The
hydrocarbons boiling above 370° C. are then passed to the second
reaction zone for additional cracking.

[0008] Conventionally, most hydrocracking processes that are implemented
for production of middle distillates and other valuable fractions retain
aromatics, e.g., boiling in the range of about 180° C. to
370° C. Aromatics boiling higher than the middle distillate range
are also included and produced in the heavier fractions.

[0009] In all of the above-described hydrocracking process configurations,
cracked products, along with partially cracked and unconverted
hydrocarbons, are passed to a distillation column for fractionating into
products including naphtha, jet fuel/kerosene and diesel boiling in the
nominal ranges of 36° C.-180° C., 180°
C.-240° C. and 240° C.-370° C., respectively, and
unconverted products boiling in the nominal range of above 370° C.
Typical jet fuel/kerosene fractions (i.e., smoke point>25 mm) and
diesel fractions (i.e., cetane number>52) are of high quality and well
above the worldwide transportation fuel specifications. Although the
hydrocracking unit products have relatively low aromaticity, aromatics
that do remain lower the key indicative properties (smoke point and
cetane number) for these products.

[0010] A need remains in the industry for improvements in hydrocracking
operations for heavy hydrocarbon feeds to produce clean transportation
fuels in an economical and efficacious manner.

SUMMARY OF THE INVENTION

[0011] In accordance with one or more embodiments, the invention relates
to systems and methods of hydrocracking heavy hydrocarbon feedstocks to
produce clean transportation fuels. An integrated hydrocracking process
includes hydroprocessing an aromatic-rich fraction of the initial feed
separately from an aromatic-lean fraction.

[0012] In a two-stage hydrocracker configuration provided herein, an
aromatic separation unit is integrated in which:

[0013] the feedstock is separated into an aromatic-rich fraction and an
aromatic-lean fraction;

[0014] the aromatic-rich fraction is passed to a first vessel of a first
stage hydroprocessing reaction zone operating under conditions effective
to hydrotreat and/or hydrocrack at least a portion of aromatic compounds
contained in the aromatic-rich fraction and to produce a first vessel
first stage hydroprocessing reaction zone effluent;

[0015] the aromatic-lean fraction is passed to a second vessel of the
first stage hydroprocessing reaction zone operating under conditions
effective to hydrotreat and/or hydrocrack at least a portion of paraffin
and naphthene compounds contained in the aromatic-lean fraction and to
produce a second vessel first stage hydroprocessing reaction zone
effluent;

[0016] a mixture of the first vessel first stage hydroprocessing reaction
zone effluent and the second vessel first stage hydroprocessing reaction
zone effluent is fractionated in a fractionating zone to produce a
product stream and a bottoms stream;

[0017] at least a portion of fractionating zone bottoms stream is passed
to a second stage hydroprocessing reaction zone to produce a second stage
hydroprocessing reaction zone effluent; and

[0018] the second stage hydroprocessing reaction zone effluent is conveyed
to the fractionating zone.

[0019] Unlike typical known methods, the present process separates the
hydrocracking feed into fractions containing different classes of
compounds with different reactivities relative to the conditions of
hydrocracking. Conventionally, most approaches subject the entire
feedstock to the same hydroprocessing reaction zones, necessitating
operating conditions that must accommodate feed constituents that require
increased severity for conversion, or alternatively sacrifice overall
yield to attain desirable process economics.

[0020] Since aromatic extraction operations typically do not provide sharp
cut-offs between the aromatics and non-aromatics, the aromatic-lean
fraction contains a major proportion of the non-aromatic content of the
initial feed and a minor proportion of the aromatic content of the
initial feed, and the aromatic-rich fraction contains a major proportion
of the aromatic content of the initial feed and a minor proportion of the
non-aromatic content of the initial feed. The amount of non-aromatics in
the aromatic-rich fraction, and the amount of aromatics in the
aromatic-lean fraction, depend on various factors as will be apparent to
one of ordinary skill in the art, including the type of extraction, the
number of theoretical plates in the extractor (if applicable to the type
of extraction), the type of solvent and the solvent ratio.

[0021] The feed portion that is extracted into the aromatic-rich fraction
includes aromatic compounds that contain heteroatoms and those that are
free of heteroatoms. Aromatic compounds that contain heteroatoms that are
extracted into the aromatic-rich fraction generally include aromatic
nitrogen compounds such as pyrrole, quinoline, acridine, carbazoles and
their derivatives, and aromatic sulfur compounds such as thiophene,
benzothiophenes and their derivatives, and dibenzothiophenes and their
derivatives. These nitrogen- and sulfur-containing aromatic compounds are
targeted in the aromatic separation step(s) generally by their solubility
in the extraction solvent. In certain embodiments, selectivity of the
nitrogen- and sulfur-containing aromatic compounds is enhanced by use of
additional stages and/or selective sorbents. Various non-aromatic
sulfur-containing compounds that may have been present in the initial
feed, i.e., prior to hydrotreating, include mercaptans, sulfides and
disulfides. Depending on the aromatic extraction operation type and/or
condition, a preferably very minor portion of non-aromatic nitrogen- and
sulfur-containing compounds can pass to the aromatic-rich fraction.

[0022] As used herein, the term "major proportion of the non-aromatic
compounds" means at least greater than 50 weight % (W %) of the
non-aromatic content of the feed to the extraction zone, in certain
embodiments at least greater than about 85 W %, and in further
embodiments greater than at least about 95 W %. Also as used herein, the
term "minor proportion of the non-aromatic compounds" means no more than
50 W % of the non-aromatic content of the feed to the extraction zone, in
certain embodiments no more than about 15 W %, and in further embodiments
no more than about 5 W %.

[0023] Also as used herein, the term "major proportion of the aromatic
compounds" means at least greater than 50 W % of the aromatic content of
the feed to the extraction zone, in certain embodiments at least greater
than about 85 W %, and in further embodiments greater than at least about
95 W %. Also as used herein, the term "minor proportion of the aromatic
compounds" means no more than 50 W % of the aromatic content of the feed
to the extraction zone, in certain embodiments no more than about 15 W %,
and in further embodiments no more than about 5 W %.

[0024] Still other aspects, embodiments, and advantages of these exemplary
aspects and embodiments, are discussed in detail below. Moreover, it is
to be understood that both the foregoing information and the following
detailed description are merely illustrative examples of various aspects
and embodiments, and are intended to provide an overview or framework for
understanding the nature and character of the claimed aspects and
embodiments. The accompanying drawings are included to provide
illustration and a further understanding of the various aspects and
embodiments, and are incorporated in and constitute a part of this
specification. The drawings, together with the remainder of the
specification, serve to explain principles and operations of the
described and claimed aspects and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The foregoing summary as well as the following detailed description
will be best understood when read in conjunction with the attached
drawings. It should be understood, however, that the invention is not
limited to the precise arrangements and apparatus shown. In the drawings
the same or similar reference numerals are used to identify to the same
or similar elements, in which:

[0026]FIG. 1 is a process flow diagram of a hydroprocessing system
operating in a two-stage configuration;

[0027]FIG. 2 is a schematic diagram of an aromatic separation apparatus;
and

[0028] FIGS. 3-8 show various examples of apparatus suitable for use as
the aromatic extraction zone.

DETAILED DESCRIPTION OF THE INVENTION

[0029] An integrated system is provided for efficient hydroprocessing of
heavy hydrocarbon feedstocks to produce clean transportation fuels. In
general, the process and apparatus described herein for producing cracked
hydrocarbons are applied to two-stage hydrocracking configurations.

[0030] An aromatic separation unit is integrated in a two-stage
hydrocracker configuration as follows:

[0031] a feedstock is separated into an aromatic-rich fraction and an
aromatic-lean fraction;

[0032] the aromatic-rich fraction is passed to a first vessel of a first
stage hydroprocessing reaction zone operating under conditions effective
to hydrotreat and/or hydrocrack at least a portion of aromatic compounds
contained in the aromatic-rich fraction and to produce a first vessel
first stage hydroprocessing reaction zone effluent;

[0033] the aromatic-lean fraction is passed to a second vessel of the
first stage hydroprocessing reaction zone operating under conditions
effective to hydrotreat and/or hydrocrack at least a portion of paraffin
and naphthene compounds contained in the aromatic-lean fraction and to
produce a second vessel first stage hydroprocessing reaction zone
effluent;

[0034] a mixture of first vessel first stage hydroprocessing reaction zone
effluent and second vessel first stage hydroprocessing reaction zone
effluent is fractionated in a fractionating zone to produce one or more
product streams and one or more bottoms streams;

[0035] at least a portion of the fractionating zone bottoms stream is
passed to a second stage hydroprocessing reaction zone to produce a
second stage hydroprocessing reaction zone effluent; and

[0036] the second stage hydroprocessing reaction zone effluent is conveyed
to the fractionating zone.

[0037]FIG. 1 is a process flow diagram of an integrated hydrocracking
apparatus 100 in the configuration of a two-stage hydrocracking unit
apparatus. Apparatus 100 includes an aromatic extraction zone 140, a
first vessel 150 of a first stage hydroprocessing reaction zone
containing a first vessel first stage hydroprocessing catalyst, a second
vessel 160 of the first stage hydroprocessing reaction zone containing a
second vessel first stage hydroprocessing catalyst, a second stage
hydroprocessing reaction zone 180 containing a second stage
hydroprocessing catalyst and a fractionating zone 170.

[0038] Aromatic extraction zone typically 140 includes a feed inlet 102,
an aromatic-rich stream outlet 104 and an aromatic-lean stream outlet
106. In certain embodiments, feed inlet 102 is in fluid communication
with fractionating zone 170 via an optional recycle conduit 120 to
receive all or a portion of the bottoms 174. Various embodiments of
and/or unit-operations contained within aromatic separation zone 140 are
described in conjunction with FIGS. 2-8.

[0039] First vessel 150 of the first stage hydroprocessing reaction zone
generally includes an inlet 151 in fluid communication with aromatic-rich
stream outlet 104 and a source of hydrogen gas via a conduit 152. First
vessel 150 of the first stage hydroprocessing reaction zone also includes
a first vessel first stage hydroprocessing reaction zone effluent outlet
154. In certain embodiments, inlet 151 is in fluid communication with
fractionating zone 170 via an optional recycle conduit 156 to receive all
or a portion of the bottoms 174.

[0040] First vessel 150 of first stage hydroprocessing reaction zone is
operated under severe conditions. As used herein, "severe conditions" are
relative and the ranges of operating conditions depend on the feedstock
being processed. In certain embodiments of the process described herein,
these conditions include a reaction temperature in the range of from
about 300° C. to 500° C., in certain embodiments from about
380° C. to 450° C.; a reaction pressure in the range of
from about 100 bars to 200 bars, in certain embodiments from about 130
bars to 180 bars; a hydrogen feed rate below about 2,500 standard liters
per liter of hydrocarbon feed (SLt/Lt), in certain embodiments from about
500 to 2,500 SLt/Lt, and in further embodiments from about 1,000 to 1,500
SLt/Lt; and a feed rate in the range of from about 0.25 h-1 to 3.0
h-1, in certain embodiments from about 0.5 h-1 to 1.0 h-1.

[0041] The catalyst used in the first vessel 150 has one or more active
metal components selected from the Periodic Table of the Elements Group
VI, VII or VIIIB. In certain embodiments the active metal component is
one or more of cobalt, nickel, tungsten and molybdenum, typically
deposited or otherwise incorporated on a support, e.g., alumina, silica
alumina, silica, or zeolites.

[0042] Second vessel 160 of the first stage hydroprocessing reaction zone
includes an inlet 161 in fluid communication with aromatic-lean stream
outlet 106 and a source of hydrogen gas via a conduit 162. Second vessel
160 of the first stage hydroprocessing reaction zone also includes a
second vessel first stage hydroprocessing reaction zone effluent outlet
164.

[0044] Second stage hydroprocessing reaction zone 180 includes an inlet
181 in fluid communication with fractionating zone bottoms stream outlet
174 and a source of hydrogen gas via a conduit 182. Second stage
hydroprocessing reaction zone 180 also includes a second stage
hydroprocessing reaction zone effluent outlet 184 that is in fluid
communication with inlet 171 of the fractionating zone 170. Note that
while one product outlet is shown, multiple product fractions can also be
recovered from fractionating zone 170.

[0045] In general, the second vessel 160 of first stage hydroprocessing
reaction zone and the second stage hydroprocessing reaction zone 180 are
operated under mild conditions. As used herein, "mild conditions" are
relative and the ranges of operating conditions depend on the feedstock
being processed. In certain embodiments of the process described herein,
these conditions include a reaction temperature in the range of from
about 300° C. to 500° C., in certain embodiments from about
330° C. to 420° C.; a reaction pressure in the range of
from about 30 bars to 130 bars, in certain embodiments from about 60 bars
to 100 bars; a hydrogen feed rate below 2,500 SLt/Lt, in certain
embodiments from about 500 to 2,500 SLt/Lt, and in further embodiments
from about 1,000 to 1,500 SLt/Lt; and a feed rate in the range of from
about 1.0 h-1 to 5.0 h-1, in certain embodiments from about 2.0
h-1 to 3.0 h-1.

[0046] The catalyst used in the second vessel of first stage
hydroprocessing reaction zone and the second stage hydroprocessing
reaction zone has one or more active metal components selected from the
Periodic Table of the Elements Group VI, VII or VIIIB. In certain
embodiments the active metal component is one or more of cobalt, nickel,
tungsten and molybdenum, typically deposited or otherwise incorporated on
a support, e.g., alumina, silica alumina, silica, or zeolites.

[0047] A feedstock is introduced via inlet 102 of the aromatic extraction
zone 140 for extraction of an aromatic-rich fraction and an aromatic-lean
fraction. Optionally, the feedstock can be combined with all or a portion
of the bottoms 174 from fractionating zone 170 via recycle conduit 120.

[0048] The aromatic-rich fraction generally includes a major proportion of
the aromatic nitrogen- and sulfur-containing compounds that were in the
initial feedstock and a minor proportion of non-aromatic compounds that
were in the initial feedstock. Aromatic nitrogen-containing compounds
that are extracted into the aromatic-rich fraction include pyrrole,
quinoline, acridine, carbazole and their derivatives. Aromatic
sulfur-containing compounds that are extracted into the aromatic-rich
fraction include thiophene, benzothiophene and its long chain alkylated
derivatives, and dibenzothiophene and its alkyl derivatives such as
4,6-dimethyl-dibenzothiophene. The aromatic-lean fraction generally
includes a major proportion of the non-aromatic compounds that were in
the initial feedstock and a minor proportion of the aromatic nitrogen-
and sulfur-containing compounds that were in the initial feedstock. The
aromatic-lean fraction is almost free of refractory nitrogen-containing
compounds, and the aromatic-rich fraction contains nitrogen-containing
aromatic compounds.

[0049] The aromatic-rich fraction discharged via outlet 104 is passed to
inlet 151 of first vessel 150 of first stage hydroprocessing reaction
zone and mixed with hydrogen gas via conduit 152. Optionally, the
aromatic-rich fraction is combined with all or a portion of the bottoms
174 from fractionating zone 170 via recycle conduit 156. Compounds
contained in the aromatic-rich fraction including aromatics compounds are
hydrotreated and/or hydrocracked. The first vessel 150 of the first stage
hydroprocessing reaction zone is operated under relatively severe
conditions. In certain embodiments, these relatively severe conditions of
the first vessel 150 are more severe than conventionally known severe
hydroprocessing conditions due to the comparatively higher concentration
of aromatic nitrogen- and sulfur-containing compounds. However, the
capital and operational costs of these more severe conditions are offset
by the reduced volume of aromatic-rich feed processed in the first vessel
150 as compared to a full range feed that would be processed in a
conventionally known severe hydroprocessing unit operation.

[0050] The aromatic-lean fraction discharged via outlet 106 is passed to
inlet 161 of the second vessel 160 of first stage hydroprocessing
reaction zone and mixed with hydrogen gas via conduit 162. Compounds
contained in the aromatic-lean fraction including paraffins and
naphthenes are hydrotreated and/or hydrocracked.

[0051] The first vessel first stage hydroprocessing reaction zone effluent
and the second vessel first stage hydroprocessing reaction zone effluent
are sent to one or more intermediate separator vessels (not shown) to
remove gases including excess H2, H2S, NH3, methane,
ethane, propane and butanes. The liquid effluents are passed to inlet 171
of the fractionating zone 170 for recovery of liquid products via outlet
172, including, for instance, naphtha boiling in the nominal range of
from about 36° C. to 180° C. and diesel boiling in the
nominal range of from about 180° C. to 370° C. It is to be
understood that the product cut points between fractions are
representative only and in practice cut points are selected based on
design characteristics and considerations for a particular feedstock. For
instance, the values of the cut points can vary by up to about 30°
C. in the embodiments described herein. In addition, it is to be
understood that while the integrated system is shown and described with
one fractionating zone 170, in certain embodiments separate fractionating
zones can be effective.

[0052] All or a portion of the bottoms can be purged via conduit 175,
e.g., for processing in other unit operations or refineries. In certain
embodiments to maximize yields and conversions a portion of bottoms 174
is recycled within the process to the aromatic separation unit 140 and/or
the first vessel 150 of first stage hydroprocessing reaction zone
(represented by dashed-lines 120 and 156, respectively).

[0053] Full or partial of fractionating zone bottoms stream discharged via
conduit 174 is mixed with hydrogen gas via inlet 182 and passed to inlet
181 of the second stage hydroprocessing reaction zone 180. The second
stage hydroprocessing reaction zone effluent is discharged via outlet 184
and processed in the fractionating zone 170.

[0054] The second vessel 160 of the first stage hydroprocessing zone and
the second stage hydroprocessing reaction zone 180 are operated under
relatively mild conditions, which can be milder than conventional mild
hydroprocessing conditions due to the comparatively lower concentration
of aromatic nitrogen- and sulfur-containing compounds thereby reducing
capital and operational costs.

[0055] In addition, either or both of the aromatic-lean fraction and the
aromatic-rich fraction also can include extraction solvent that remains
from the aromatic extraction zone 140. In certain embodiments, extraction
solvent can be recovered and recycled, e.g., as described with respect to
FIG. 2.

[0056] Further, in certain embodiments aromatic compounds without
heteroatoms (e.g., benzene, toluene and their derivatives) are passed to
the aromatic-rich fraction and are hydrogenated and hydrocracked in the
first vessel of the first stage (relatively more severe) hydrocracking
zone to produce light distillates. The yield of these light distillates
that meet the product specification derived from the aromatic compounds
without heteroatoms is greater than the yield in conventional
hydrocracking operations due to the focused and targeted hydrocracking
zones.

[0057] In the above-described embodiment, the feedstock generally includes
any liquid hydrocarbon feed conventionally suitable for hydrocracking
operations, as is known to those of ordinary skill in the art. For
instance, a typical hydrocracking feedstock is vacuum gas oil (VGO)
boiling in the nominal range of from about 300° C. to 900°
C. and in certain embodiments in the range of from about 370° C.
to 520° C. De-metalized oil (DMO) or de-asphalted oil (DAO) can be
blended with VGO or used as is. The hydrocarbon feedstocks can be derived
from naturally occurring fossil fuels such as crude oil, shale oils, or
coal liquids; or from intermediate refinery products or their
distillation fractions such as naphtha, gas oil, coker liquids, fluid
catalytic cracking cycle oils, residuals or combinations of any of the
aforementioned sources. In general, aromatics content in VGO feedstock is
in the range of from about 15 to 60 volume % (V %). The recycle stream
can include 0 W % to about 80 W % of stream 174, in certain embodiments
about 10 W % to 70 W % of stream 174 and in further embodiments about 20
W % to 60 W % of stream 174, for instance, based on conversions in each
zone of between about 10 W % and 80 W %.

[0058] The aromatic separation apparatus is generally based on selective
aromatic extraction. For instance, the aromatic separation apparatus can
be a suitable solvent extraction aromatic separation apparatus capable of
partitioning the feed into a generally aromatic-lean stream and a
generally aromatic-rich stream. Systems including various established
aromatic extraction processes and unit operations used in other stages of
various refinery and other petroleum-related operations can be employed
as the aromatic separation apparatus described herein. In certain
existing processes, it is desirable to remove aromatics from the end
product, e.g., lube oils and certain fuels, e.g., diesel fuel. In other
processes, aromatics are extracted to produce aromatic-rich products, for
instance, for use in various chemical processes and as an octane booster
for gasoline.

[0059] As shown in FIG. 2, an aromatic separation apparatus 240 can
include suitable unit operations to perform a solvent extraction of
aromatics, and recover solvents for reuse in the process. A feed 202 is
conveyed to an aromatic extraction vessel 208 in which in which a first,
aromatic-lean, fraction is separated as a raffinate stream 210 from a
second, generally aromatic-rich, fraction as an extract stream 212. A
solvent feed 215 is introduced into the aromatic extraction vessel 208.

[0060] A portion of the extraction solvent can also exist in stream 210,
e.g., in the range of about 0 W % to about 15 W % (based on the total
amount of stream 210), in certain embodiments less than about 8 W %. In
operations in which the solvent existing in stream 210 exceeds a desired
or predetermined amount, solvent can be removed from the hydrocarbon
product, for example, using a flashing or stripping unit 213, or other
suitable apparatus. Solvent 214 from the flashing unit 213 can be
recycled to the aromatic extraction vessel 208, e.g., via a surge drum
216. Initial solvent feed or make-up solvent can be introduced via stream
222. An aromatic-lean stream 206 is discharged from the flashing unit
213.

[0061] In addition, a portion of the extraction solvent can also exist in
stream 212, e.g., in the range of about 70 W % to about 98 W % (based on
the total amount of stream 215), in certain embodiments less than about
85 W %. In embodiments in which solvent existing in stream 212 exceeds a
desired or predetermined amount, solvent can be removed from the
hydrocarbon product, for example, using a flashing or stripping unit 218
or other suitable apparatus. Solvent 221 from the flashing unit 218 can
be recycled to the aromatic extraction vessel 208, e.g., via the surge
drum 216. An aromatic-rich stream 204 is discharged from the flashing
unit 218.

[0062] Selection of solvent, operating conditions, and the mechanism of
contacting the solvent and feed permit control over the level of aromatic
extraction. For instance, suitable solvents include furfural,
N-methyl-2-pyrrolidone, dimethylformamide, dimethylsulfoxide, phenol,
nitrobenzene, sulfolanes, acetonitrile, furfural, or glycols, and can be
provided in a solvent to oil ratio of about 20:1, in certain embodiments
about 4:1, and in further embodiments about 1:1. Suitable glycols include
diethylene glycol, ethylene glycol, triethylene glycol, tetraethylene
glycol and dipropylene glycol. The extraction solvent can be a pure
glycol or a glycol diluted with from about 2 to 10 W % water. Suitable
sulfolanes include hydrocarbon-substituted sulfolanes (e.g., 3-methyl
sulfolane), hydroxy sulfolanes (e.g., 3-sulfolanol and
3-methyl-4-sulfolanol), sulfolanyl ethers (i.e., methyl-3-sulfolanyl
ether), and sulfolanyl esters (e.g., 3-sulfolanyl acetate).

[0063] The aromatic separation apparatus can operate at a temperature in
the range of from about 20° C. to 200° C., and in certain
embodiments in the range of from about 40° C. to 80° C. The
operating pressure of the aromatic separation apparatus can be in the
range of from about 1 bar to 10 bars, and in certain embodiments in the
range of about 1 bar to 3 bars. Types of apparatus useful as the aromatic
separation apparatus in certain embodiments of the system and process
described herein include stage-type extractors or differential
extractors.

[0064] An example of a stage-type extractor is a mixer-settler apparatus
340 schematically illustrated in FIG. 3. Mixer-settler apparatus 340
includes a vertical tank 381 incorporating a turbine or a propeller
agitator 382 and one or more baffles 384. Charging inlets 386, 388 are
located at the top of tank 381 and outlet 391 is located at the bottom of
tank 381. The feedstock to be extracted is charged into vessel 381 via
inlet 386 and a suitable quantity of solvent is added via inlet 388. The
agitator 382 is activated for a period of time sufficient to cause
intimate mixing of the solvent and charge stock, and at the conclusion of
a mixing cycle, agitation is halted and, by control of a valve 392, at
least a portion of the contents are discharged and passed to a settler
394. The phases separate in the settler 394 and a raffinate phase
containing an aromatic-lean hydrocarbon mixture and an extract phase
containing an aromatic-rich mixture are withdrawn via outlets 396 and
398, respectively. In general, a mixer-settler apparatus can be used in
batch mode, or a plurality of mixer-settler apparatus can be staged to
operate in a continuous mode.

[0065] Another stage-type extractor is a centrifugal contactor.
Centrifugal contactors are high-speed, rotary machines characterized by
relatively low residence time. The number of stages in a centrifugal
device is usually one, however, centrifugal contactors with multiple
stages can also be used. Centrifugal contactors utilize mechanical
devices to agitate the mixture to increase the interfacial area and
decrease the mass transfer resistance.

[0066] Various types of differential extractors (also known as "continuous
contact extractors,") that are also suitable for use as an aromatic
extraction apparatus include, but are not limited to, centrifugal
contactors and contacting columns such as tray columns, spray columns,
packed towers, rotating disc contactors and pulse columns.

[0067] Contacting columns are suitable for various liquid-liquid
extraction operations. Packing, trays, spray or other droplet-formation
mechanisms or other apparatus are used to increase the surface area in
which the two liquid phases (i.e., a solvent phase and a hydrocarbon
phase) contact, which also increases the effective length of the flow
path. In column extractors, the phase with the lower viscosity is
typically selected as the continuous phase, which, in the case of an
aromatic extraction apparatus, is the solvent phase. In certain
embodiments, the phase with the higher flow rate can be dispersed to
create more interfacial area and turbulence. This is accomplished by
selecting an appropriate material of construction with the desired
wetting characteristics. In general, aqueous phases wet metal surfaces
and organic phases wet non-metallic surfaces. Changes in flows and
physical properties along the length of an extractor can also be
considered in selecting the type of extractor and/or the specific
configuration, materials or construction, and packing material type and
characteristics (i.e., average particle size, shape, density, surface
area, and the like).

[0068] A tray column 440 is schematically illustrated in FIG. 4. A light
liquid inlet 488 at the bottom of column 440 receives liquid hydrocarbon,
and a heavy liquid inlet 491 at the top of column 440 receives liquid
solvent. Column 440 includes a plurality of trays 481 and associated
downcomers 482. A top level baffle 484 physically separates incoming
solvent from the liquid hydrocarbon that has been subjected to prior
extraction stages in the column 440. Tray column 440 is a multi-stage
counter-current contactor. Axial mixing of the continuous solvent phase
occurs at region 486 between trays 481, and dispersion occurs at each
tray 481 resulting in effective mass transfer of solute into the solvent
phase. Trays 481 can be sieve plates having perforations ranging from
about 1.5 to 4.5 mm in diameter and can be spaced apart by about 150-600
mm.

[0069] Light hydrocarbon liquid passes through the perforation in each
tray 481 and emerges in the form of fine droplets. The fine hydrocarbon
droplets rise through the continuous solvent phase and coalesce into an
interface layer 496 and are again dispersed through the tray 481 above.
Solvent passes across each plate and flows downward from tray 481 above
to the tray 481 below via downcomer 482. The principal interface 498 is
maintained at the top of column 440. Aromatic-lean hydrocarbon liquid is
removed from outlet 492 at the top of column 440 and aromatic-rich
solvent liquid is discharged through outlet 494 at the bottom of column
440. Tray columns are efficient solvent transfer apparatus and have
desirable liquid handling capacity and extraction efficiency,
particularly for systems of low-interfacial tension.

[0070] An additional type of unit operation suitable for extracting
aromatics from the hydrocarbon feed is a packed bed column. FIG. 5 is a
schematic illustration of a packed bed column 540 having a hydrocarbon
inlet 591 and a solvent inlet 592. A packing region 588 is provided upon
a support plate 586. Packing region 588 comprises suitable packing
material including, but not limited to, Pall rings, Raschig rings,
Kascade rings, Intalox saddles, Berl saddles, super Intalox saddles,
super Berl saddles, Demister pads, mist eliminators, telerrettes, carbon
graphite random packing, other types of saddles, and the like, including
combinations of one or more of these packing materials. The packing
material is selected so that it is fully wetted by the continuous solvent
phase. The solvent introduced via inlet 592 at a level above the top of
the packing region 588 flows downward and wets the packing material and
fills a large portion of void space in the packing region 588. Remaining
void space is filled with droplets of the hydrocarbon liquid which rise
through the continuous solvent phase and coalesce to form the
liquid-liquid interface 598 at the top of the packed bed column 540.
Aromatic-lean hydrocarbon liquid is removed from outlet 594 at the top of
column 540 and aromatic-rich solvent liquid is discharged through outlet
596 at the bottom of column 540. Packing material provides large
interfacial areas for phase contacting, causing the droplets to coalesce
and reform. The mass transfer rate in packed towers can be relatively
high because the packing material lowers the recirculation of the
continuous phase.

[0071] Further types of apparatus suitable for aromatic extraction in the
system and method herein include rotating disc contactors. FIG. 6 is a
schematic illustration of a rotating disc contactor 640 known as a
Scheiebel® column commercially available from Koch Modular Process
Systems, LLC of Paramus, N.J., USA. It will be appreciated by those of
ordinary skill in the art that other types of rotating disc contactors
can be implemented as an aromatic extraction unit included in the system
and method herein, including but not limited to Oldshue-Rushton columns,
and Kuhni extractors. The rotating disc contactor is a mechanically
agitated, counter-current extractor. Agitation is provided by a rotating
disc mechanism, which typically runs at much higher speeds than a turbine
type impeller as described with respect to FIG. 3.

[0072] Rotating disc contactor 640 includes a hydrocarbon inlet 691 toward
the bottom of the column and a solvent inlet 692 proximate the top of the
column, and is divided into number of compartments formed by a series of
inner stator rings 682 and outer stator rings 684. Each compartment
contains a centrally located, horizontal rotor disc 686 connected to a
rotating shaft 688 that creates a high degree of turbulence inside the
column. The diameter of the rotor disc 686 is slightly less than the
opening in the inner stator rings 682. Typically, the disc diameter is
33-66% of the column diameter. The disc disperses the liquid and forces
it outward toward the vessel wall 698 where the outer stator rings 684
create quiet zones where the two phases can separate. Aromatic-lean
hydrocarbon liquid is removed from outlet 694 at the top of column 640
and aromatic-rich solvent liquid is discharged through outlet 696 at the
bottom of column 640. Rotating disc contactors advantageously provide
relatively high efficiency and capacity and have relatively low operating
costs.

[0073] An additional type of apparatus suitable for aromatic extraction in
the system and method herein is a pulse column. FIG. 7 is a schematic
illustration of a pulse column system 740, which includes a column with a
plurality of packing or sieve plates 788, a light phase, i.e., solvent,
inlet 791, a heavy phase, i.e., hydrocarbon feed, inlet 792, a light
phase outlet 794 and a heavy phase outlet 796.

[0074] In general, pulse column system 740 is a vertical column with a
large number of sieve plates 788 lacking down corners. The perforations
in the sieve plates 788 typically are smaller than those of non-pulsating
columns, e.g., about 1.5 mm to 3.0 mm in diameter.

[0075] A pulse-producing device 798, such as a reciprocating pump, pulses
the contents of the column at frequent intervals. The rapid reciprocating
motion, of relatively small amplitude, is superimposed on the usual flow
of the liquid phases. Bellows or diaphragms formed of coated steel (e.g.,
coated with polytetrafluoroethylene), or any other reciprocating,
pulsating mechanism can be used. A pulse amplitude of 5-25 mm is
generally recommended with a frequency of 100-260 cycles per minute. The
pulsation causes the light liquid (solvent) to be dispersed into the
heavy phase (oil) on the upward stroke and heavy liquid phase to jet into
the light phase on the downward stroke. The column has no moving parts,
low axial mixing, and high extraction efficiency.

[0076] A pulse column typically requires less than a third the number of
theoretical stages as compared to a non-pulsating column. A specific type
of reciprocating mechanism is used in a Karr Column which is shown in
FIG. 8.

[0077] Distinct advantages are offered by the selective hydrocracking
apparatus and processes described herein when compared to conventional
processes for hydrocracking selected fractions. Aromatics across a full
range of boiling points contained in heavy hydrocarbons are extracted and
separately processed in hydroprocessing reaction zone operating under
conditions optimized for hydrotreating and/or hydrocracking aromatics,
including aromatic nitrogen compounds that are prone to deactivate the
hydrotreating catalyst.

[0078] According to the present processes and apparatus, the overall
middle distillate yield is improved as the initial feedstock is separated
into aromatic-rich and aromatic-lean fractions and hydrotreated and/or
hydrocracked in different hydroprocessing reaction zones operating under
conditions optimized for each fraction.

Example

[0079] A sample of vacuum gas oil (VGO) derived from Arab light crude oil
was extracted in an extractor. Furfural was used as the extractive
solvent. The extractor was operated at 60° C., atmospheric
pressure, and at a solvent to diesel ratio of 1.1:1.0. Two fractions were
obtained: an aromatic-rich fraction and an aromatic-lean fraction. The
aromatic-lean fraction yield was 52.7 W % and contained 0.43 W % of
sulfur and 5 W % of aromatics. The aromatic-rich fraction yield was 47.3
W % and contained 95 W % of aromatics and 2.3 W % of sulfur. The
properties of the VGO, aromatic-rich fraction and aromatic-lean fraction
are given in Table 1.

[0080] The aromatic-rich fraction was hydrotreated in a fixed-bed
hydrotreating unit containing Ni--Mo on silica alumina as hydrotreating
catalyst at 150 Kg/cm2 hydrogen partial pressure, 400° C.,
liquid hourly space velocity of 1.0 h-1 and a hydrogen feed rate of
1,000 SLt/Lt. The Ni--Mo on alumina catalyst was used to desulfurize and
denitrogenize the aromatic-rich fraction, which includes a significant
amount of the nitrogen content originally contained in the feedstock. The
conversion of hydrocarbons boiling above 370° C. was 60 W %.

[0081] The aromatic-lean fraction was hydrotreated in a fixed-bed
hydrotreating unit containing Ni--Mo on silica alumina as hydrotreating
catalyst at 70 Kg/cm2 hydrogen partial pressure, 370° C.,
liquid hourly space velocity of 1.0 h-1 and a hydrogen feed rate of
1,000 SLt/Lt. The Ni--Mo on alumina catalyst was used to desulfurize and
denitrogenize the aromatic lean fraction. The conversion of hydrocarbons
boiling above 370° C. was 15 W %.

[0082] The total reactor effluents from the first stage reactors were
combined and sent to the second stage reactor for further cracking of the
unconverted bottoms. The second stage reactor contains a zeolitic
hydrocracking base catalyst designed for middle distillate selectivity
(Ni--Mo on USY Ti--Zr inserted zeolite catalyst) at 120 Kg/cm2
hydrogen partial pressure, 393° C., a liquid hourly space velocity
of 1.25 h-1 and a hydrogen feed rate of 1,000 SLt/Lt.

[0083] The once-thru conversion in the second reactor was 50 W % and 30 W
% (of feedstock) of bottoms were recycled back to the fractionator. The
product yields resulting from the each hydroprocesser and the integrated
process are given in Table 2:

[0084] The method and system herein have been described above and in the
attached drawings; however, modifications will be apparent to those of
ordinary skill in the art and the scope of protection for the invention
is to be defined by the claims that follow.